1. Field of the Invention
The invention concerns the determination of time-dependent dephasing factors of at least one spectral component of at least two spectral components in a region of interest in an object under examination by magnetic resonance (MR) test measurements, and a method for using the determined dephasing factors. The invention also concerns a magnetic resonance system and a non-transitory, electronically readable data carrier for implementing such a method.
2. Description of the Prior Art
Magnetic resonance (MR) is a known modality with which images of the interior of an object under examination can be generated. In simple terms, the object under examination is positioned in a magnetic resonance scanner in a strong static homogeneous basic magnetic field, also called a B0 field, with field strengths of 0.2 tesla to 7 tesla and more so that the nuclear spins of the object are oriented along the basic magnetic field. To excite magnetic resonance of the nuclear spins, radio-frequency excitation pulses (RF pulses) are radiated into the object under examination, the excited nuclear spins emit signals that are measured as so-called k-space data, which are used as the basis for the reconstruction of MR images or the determination of spectroscopy data. Rapidly switched magnetic gradient fields are superimposed on the basic magnetic field for spatially encoding the measured data. The measured data recorded are digitized and stored as complex numbers in a so-called k-space matrix. An associated MR image can be reconstructed from the k-space matrix populated with these values, for example by a multidimensional Fourier transformation.
During MR measurements of nuclear spins, it is possible to separate spectral components included in MR data. The spectral components can designate different spin species, for example nuclear spins in a fat environment and in an aqueous environment. To this end, often so-called chemical shift imaging multi-echo MR measurement sequences are used within the context of Dixon techniques. Such techniques typically make use of the fact that the resonance frequency of nuclear spins depends on the molecular and/or chemical environment. This effect is known as the chemical shift. Hence, different spin species have different resonance frequencies from which the measured spectrum of the MR data is composed. For example, the difference between two resonance frequencies of different spectral components can be expressed in ppm (“parts per million”, i.e. 10−6).
Many chemical species, for example water, have monofrequency MR spectra. Others, such as fat, have a non-monofrequency MR spectrum. Others have multiple, coupled resonances with a known amplitude ratio, a known phase position, if applicable, and known frequency differences. This advance knowledge can be utilized during the determination of the total signal from these species, see for example Provencher et al. “SW. Estimation of metabolite concentrations from localized in vivo proton NMR spectra” MRM 30: 672 (1993).
The MR signal of hydrogen nuclear spins in water is often considered to be a first spectral component and that of hydrogen nuclear spins in fatty acid chains to be a second spectral component. In such a case, MR data can be used to determine a water MR image and a fat MR image, i.e. in each case individual MR images of the two spectral components. This is of interest for a wide variety of clinical and/or medical applications.
In order to be able to separate the spectral components from one another, in the context of the Dixon technique, MR signals are acquired at a number of echo times after the excitation. The combined MR signals form the MR data. The different spectral components have different phase positions and amplitudes at the different echo times. Taking this effect into account enables the quantities of the chemical species to be determined separately.
For this purpose, a signal model is usually used that links the measured or acquired MR data with different physically relevant variables. The different variables can be the different spectral components to be determined, the spectra thereof, and—depending upon the precision, scope and complexity of the signal model—further unknown aspects of the measuring system. This can enable the spectral components that are taken into account in the signal model to be determined for each image element of the MR data.
The spectral model for fat as a spectral component is, for example, known from Hamilton G. et al. “In vivo characterization of the liver fat 1H MR spectrum” NMR Biomed. 24: 784-790 (2011).
However, the results can vary depending upon the spectral model that is selected, because in each case different assumptions can be made regarding the properties of the underlying spectrum of the fat.
Although the fat spectra can be calibrated individually in accordance with the procedure of Hamilton et al. in order for this procedure to be used as part of the signal model for Dixon techniques, a high time expenditure and a high degree of expertise and experience are required. In addition, in the case of Dixon techniques with only a few echo times, the fat spectrum is only evaluated for a correspondingly low number of complex-valued dephasing factors (phase position and amplitude) in the time range.